Since our brains remain the same now as they were then, the genetic argument cannot be right either.

And of course as a child gets half its genes from its father and half from its mother, the refrain in the programme of "like father like son" is a crass genetic error.

If Liam has ADHD (if that were a sensible diagnosis, which many child psychologists would doubt), and if ADHD were a genetic condition (for which there is no strong evidence), then it would be as likely to be inherited from his mother as his father.

External factors

If there is more violent crime today we have to look for other reasons than inside peoples' brains.

Guns and drugs other than alcohol and nicotine are more available, there is a growing gap between rich and poor, and life in sink estates, perhaps especially for young men, offers little hope.

But despite all this, one might argue that maybe there is something different about the brains or the genes of someone prepared to be violent, and that a genetic test or a brain imaging, as in the programme, might reveal it.

This is only true up to a point.

There is some evidence that children with a particular genetic mutation, if brought up in an abusive environment, are more likely to be violent or abusive in turn when they become adult.

But the crucial factor is the interaction between gene (which is rare) and environment (which is common), so if we want to do something about it we should try to prevent any child, with or without any particular gene, from being abused.

'Beware'

There is also some evidence that some people diagnosed as psychopathic may show particular brain abnormalities.

But many people so diagnosed do not show such abnormalities, and many people who are behaviourally not violent do show them.

Neuroscience can say little about the causes or the treatment of violence in our society